1. Field of the Invention
The present invention relates to a laser beam machining apparatus for laser beam machining of a work such as a semiconductor wafer, and particularly to a laser beam machining apparatus in which the shape of a condensed spot of a laser beam can be controlled.
2. Description of the Related Art
In a semiconductor device manufacturing process, a plurality of regions are demarcated in a surface of a substantially circular disk-shaped semiconductor wafer by planned dividing lines called streets which are arranged in a lattice pattern, and devices such as ICs and LSIs are formed in the demarcated regions. Then, the semiconductor wafer is cut (diced) along the planned dividing lines to divide the regions with the devices formed therein, whereby individual semiconductor chips are manufactured. Similarly, an optical device wafer in which light receiving devices such as photodiodes or light emitting devices such as laser diodes are stacked on a surface of a sapphire substrate is also cut (diced) along streets, thereby dividing the wafer into individual devices such as photodiodes, laser diodes, etc., which are widely utilized for electric apparatuses.
As a method for dividing the wafer such as the above-mentioned semiconductor wafer and optical device wafer along the streets, there has been proposed a method in which irradiation with a pulsed laser beam is conducted along the streets formed in the wafer so as to form laser beam-machined grooves, and the wafer is broken along the laser beam-machined grooves (refer to, for example, Japanese Patent Laid-open No. 2004-9139).
In using the laser beam with which to irradiate the work, the machining conditions can be appropriately controlled, as required, by regulating the output, wavelength, repetition frequency, condensed beam spot shape, etc. of the laser beam. However, it is difficult to appropriately change the condensed beam spot shape into a circle or ellipses varying in the ratio of major axis to minor axis, as required, so that there are limitations to the control of the machining conditions.
In consideration of the above circumstances, the present applicant has proposed, in Japanese Patent Application No. 2005-331118, a laser beam machining apparatus in which a condenser for condensing a laser beam includes a first cylindrical lens and a second cylindrical lens oriented to have a condensing direction orthogonal to that of the first cylindrical lens, and modification of the condensed beam spot shape of the laser beam into a circle or ellipses varying in the ratio of major axis to minor axis can be easily achieved by regulating the interval between the first cylindrical lens and the second cylindrical lens.
In this laser beam machining apparatus, however, since the condenser is so configured to change the condensed beam spot shape through the combination of cylindrical lenses, the shape of the circular beam spot becomes close to a square due to the influence of aberration. Therefore, in the case of boring a via hole in a semiconductor device, for example, it is impossible to form a circular via hole. In addition, in the condenser based on the combination of cylindrical lenses as above-mentioned, the condensed beam spot shape is either a circle or an ellipse. Therefore, machining with an elliptic condensed beam spot and machining with a circular condensed beam spot cannot be carried out at the same time.